I Field of the Invention
The present invention relates to a radio transmission system for communications between a fixed station and a number of fixed subscriber stations that operate independently of one another.
II Description of the Known Art
For information transmission in systems with a radial operating structure which are characterized by a local base station and a plurality of noninterdependent subscriber stations, wherein the transmission of information is performed each time from the base station to the individual subscriber stations and from the subscriber stations to the base station, different transmission processes are used.
It is characteristic of such systems that, as a rule, access by the individual subscriber stations is limited to no more than one of a great number of information channels at a time. These information channels are combined by multiplex processes. Code-multiplex, time-multiplex and frequency-multiplex processes are known.
In the code-multiplex process the information is modulated by base modulation on a carrier, and the resultant signal, which is a narrow-band signal in comparison to the channel bandwidth, is spread out spectrally to the channel bandwidth by multiplex modulation by means of a code word identifying the receiver.
The recognition of the signal is performed with the aid of the spectral coding. The message is selected in the receiver from among the plurality of spectrally coded messages superimposed in the code multiplex channel, with the aid of the code word associated with it.
In the time-multiplex process the entire bandwidth of a radio channel is available to each subscriber, which the latter may use only at definite, short time intervals. The characters or groups of characters identifying the different subscribers are contained one inside the other and are transmitted at a correspondingly higher bit rate in the single radio channel, while the time channel associated with each subscriber is repeated periodically with the length of the frame period.
Code-multiplex and time-multiplex processes are used preferentially for digital information transmission systems.
By their use virtually any signal-to-noise ratio can be achieved in exchange for a great transmission bandwidth.
The code- and time-multiplex processes require very efficient, and at present still expensive digital signal processing, especially in the subscriber stations, but also in the base station. In the time-multiplex process the necessary line synchronization is complicated, and, in the code-multiplex process especially, so is the code synchronization in the subscriber stations.
In the frequency-multiplex process the total bandwidth available for the transmission of information is divided up into narrow frequency bands which correspond each to one information transmission channel. Such a narrow frequency band is available to the subscriber throughout the radio transmission. Frequency-multiplex processes can be used to advantage in analog or combination digital-analog systems requiring only medium quality because an established cost-effective technology is available and only a relatively small transmission bandwidth is needed.
For the use of the above-described multiplex transmission processes in most of the systems of this kind known heretofore, especially radio transmission systems, the same processes have been used in both directions of information transmission, i.e., both from the base station to the subscriber stations and also in the opposite direction.
Such a system is described, for example, in the German patent specification DE-OS 2537683. The individual radio channels are operated as duplex channels in the lower and upper sideband with a given duplex separation, all channels being disposed in the frequency multiplex and are all at the same channel separation from one another.
Likewise, transmission processes of this kind are known from carrier frequency technology and from radio beam technology. In contrast to the systems herein considered, however, a point-to-point information transfer is performed between two fixed base stations, a relatively great number of information channels being always beamed in transmission.
German patent specification DE-OS 3447107 discloses a digital information transmission process for cellular networks, in which a different modulation process is used in each direction in the information transmission channel. For the transmission of information the mobile subscribers access one of a number of information channels.
In the direction from the fixed base station to the subscriber stations, each information channel is spread by spread spectrum modulation. The spread information channels are superimposed on one another and the broad-band sum signal thus obtained is transmitted in a common frequency band.
The transmission of information from the subscriber stations to the base station is performed in narrow-band frequency channels separate from one another.
Code multiplex is favored as a multiplex process and is combined with a binary spread modulation. This process has advantages especially in regard to security against interference in mobile radio networks. The spread modulation that is concretely to be used can be varied before transmission starts.
Various measures, such as the use of different kinds of spread modulation, different frequency bands, different time channels, limited channel numbers and channel frequencies serve for interference suppression between adjacent radio cells.
German patent specification DE-OS 3527331 discloses a transmission process which uses different combinations of multiplex processes in both directions of transmission of the digital radio system for the beamed transmission of information channels.
In the base station to subscriber station transmission the information to be sent is inserted into the channel using the combination of code-multiplex, time-multiplex and frequency-multiplex processes which is demultiplexed by the same process in the subscriber station.
Transmission from the subscriber stations to the base station is performed in narrow-band frequency channels separate from one another, or in time channels, or in code planes separate from one another, or combinations thereof.
When traffic is limited, beaming methods are proposed, with the exclusive use of time-multiplexing or a combination of time-multiplexing and code-multiplexing.
Conversion to carrier frequencies is also being discussed.